1. Field of the Invention
This invention relates to the field of products manufactured from fiber-reinforced elastomeric resin, particularly to techniques for fabricating such products, especially rotor blades formed in one cycle at elevated temperature and pressure.
2. Description of the Prior Art
Conventional helicopter aerodynamic rotor blades (main rotor and tail rotor), fabricated of composite materials and having fiber-reinforced elastomeric resign, require multiple cure cycles at elevated temperature and pressure lasting several hours in order to form the shape of the components of the blade assembly. During these cure cycles, the resin of the composite material polymerizes to optimal structural stiffness and strength, provided temperature is controlled and varied over the length of the cycle in accordance with a defined schedule usually set by the manufacturer's specifications. Positive pressure applied to the material helps to force air from the components to ambient atmosphere outside the tool in which the components are formed.
This technique requires carefully sizing many laminae of composite material in a precured condition cut from raw stock to precise dimensions. Each component requires a molding tool, into which precured laminae are placed, sealed by a plastic bag enveloping the tool, and pressurized during the cure cycle. Usually the components are cured in an autoclave/or large furnace having a sealed chamber whose pressure is increased in order to induce air to escape the laminae during the cure cycle. This process requires careful control of temperature and pressure to the requirements of the cure cycle. Typically temperature is raised to 250 degrees F and pressure to 85 psi.
U.S. Pat. No. 4,095,322 describes a helicopter main rotor blade fabricated from composite material of fiber-reinforced resin. The method of fabrication requires several cure cycles at high temperature and pressure to form individual components and a final cure cycle during which the components are held in position and bonded to form an assembly. The structure of the rotor blade and aft fairing, a subassembly of the blade assembly, described in U.S. Pat. No. 4,316,701 are compatible with the fabrication method of the '322 patent.
However, various resins and bonds produced by different manufacturers require different cure cycle conditions to satisfy manufacturer's specifications and to realize optimal structural properties. A fabricator using diverse materials must carefully control operation of the autoclave so that products being cured and/or bonded there are compatible with the temperature cycle being used.
A helicopter tail rotor blade, or fan blade, fabricated in the conventional way includes in its essential elements a leading edge cap forming the nose or leading edge of the airfoil, a spar assembly and a trailing edge. Each of these components requires a separate cure and molding tool to produce acceptable dimensional tolerance control and physical properties. After these components are fabricated, they are bonded to other components to form subassemblies, which are subsequently placed in a molding tool or fixture and bonded to other subassemblies while other components are being cured.
For example, the spar packs are bonded to the leading edge cap to produce a subassembly comprising the forward portion of the airfoil contour. Thereafter, the spar-leading edge subassembly is bonded to the spare core subassembly to form the major portion of the blade, exclusive of the trailing edge. A trailing edge block is fabricated separately and then bonded to the rearward edge of the core, or formed and bonded integrally with the core in a single cure cycle.
Time, complexity and cost to produce rotor blades using multiple cure and bonding cycles are prohibitive.